This invention generally relates to transfer printing and more particularly to a machine for transfer printing figures, letters, other symbols or designs on plastic articles such as molded plastic key caps for business-machine and other typwriter-like keyboards.
There is disclosed in Japanese Patent Applications No. 57-145291 a method of transfer printing figures, letters and/or other symbols accurately and precisely at required positions on the surfaces of molded plastic articles by a simple process and in such a manner that the transfer-printed image on the plastic surface remains highly resistive to abrasion and/or scratching. This method employs a transfer sheet which comprises a substrate and a pattern layer formed thereon with thermo-diffusible colorants. The pattern comprises one or more figures, letters, other symbols or designs or a combination thereof, which will be referred to as a pattern or image when necessary for simplicity of description.
According to this method, molded plastic articles each having a surface to be transfer-printed are arranged in any array and heated to a predetermined temperature, and a transfer sheet is placed over the array, with the pattern layer of the transfer sheet being in contact with the surfaces of the articles to be transfer-printed. Then a pad having a plurality of projections made of elastic material and each having a curved outer surface is pressed against the transfer sheet so as to bring the pattern layer thereon into closer contact with the surface of the articles so that the images on the transfer sheet are transferred onto the surfaces of the articles, whereupon the pad and then the transfer sheet are removed from on the articles.
The above method has proved to be particularly useful in transfer printing on articles such as molded plastic key caps for the keyboards of typewriters or other business machines, which requires accurate positioning of the transfer sheet relative to the articles for transfer printing.
For carrying out the above-mentioned method, however, it has been impossible to use various known printing machines as they are chiefly for the following reasons. In the above-mentioned transfer printing method, the articles to be transfer-printed must be heated before a transfer sheet is placed on the surfaces of the articles to be transfer-printed. With the conventional machine, when the articles to be transfer-printed are heated, the frame or holder in which the articles are set or supported is also heated at the same time and thermally expands so that the articles held therein are displaced from the originally set positions and it becomes impossible to bring the pattern on the transfer sheet into accurate registration with the surface of the articles to be transfer-printed.
Accordingly, the primary object of the invention is to provide a transfer printing machine which is capable of conducting transfer printing accurately and efficiently.
Another object of the invention is to provide such a transfer printing machine as aforesaid which is capable of transfer printing figures, letters and/or other symbols or designs on a plurality of articles, particularly molded plastic objects such as key caps for typewriters or the like business machines simultaneously in a single shot without intolerable displacement of the printed images from their proper position on the articles.
The invention will be described in detail with reference to the accompanying drawings.